Metal powders are used in industry for the manufacture of metal products by compacting the metal powder in a die under high pressures, ejecting the compact from the die and optionally sintering the product. In the majority of powder metallurgical (PM) applications a lubricant is comprised in the powder in order to provide the necessary lubrication action between powder particles during compaction and between the die and the compact during ejection from the die. Lubrication achieved by a lubricant included in the metal powder is referred to as internal lubrication in contrast to external lubrication, which is achieved by applying a lubricant to the walls of the die, wherein the powder is compacted. Insufficient lubrication during ejection results in excessive friction between the compact and the die resulting in high ejection energies and damage of die surfaces and product surfaces.
Internal lubrication is achieved by using special lubricants. Normally these lubricants are admixed with the iron or iron-based powder in the form of a powder. Some lubricants may also be used for binding additives, such as e.g. alloying elements, to the iron or iron-based particles. In these cases the lubricants thus work as binding agents and reduce or eliminate segregation of the additives during shipping and handling.
Commonly used lubricants for PM applications are metal soaps, such as lithium and zinc stearate. A disadvantage with this type of lubricant is that oxides of the metals in the lubricant contaminate the inside of the sintering furnace as a result of release of metals from the lubricant during sintering, another problem is that stains may be formed on the component after sintering. Another commonly used lubricant is ethylene bis stearamide (EBS). Stains may also be formed on the component after sintering when using this lubricant, but to a lesser extent compared with using e.g. zinc stearate. As lubricants strongly affect compacting and sintering properties of metal powders optimization of amount, composition and structure of the used lubricant is of vital importance to obtain high and consistent densities and good surface finishes of the produced parts.